Double-sided holographic replicas

ABSTRACT

A double-sided, holographic replica comprises a web of transparent, thermoplastic, sheet material having a separate series of off-axis-recorded, phase-modulated holograms pressed into each of the opposite surfaces, respectively, of the web. Oppositely disposed (front and rear) holograms are such as to provide angularly displaced reconstructed images, respectively, when a monochromatic read-out beam is transmitted through the web. The double-sided holographic replica is made by first pressing the web and a first holographic master between a first pair of heated calender rollers to form one replica on one surface of the web, and then pressing the web, while thermoplastically adhered to the first master, and a second holographic master between a second pair of heated calender rollers to form another replica on the other surface of the web. The first and second masters are subsequently cooled and separated from the web.

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[45] Feb. 5, 1974 1 1 DOUBLE-SIDED HOLOGRAPHIC REPLICAS [75] Inventors:William James Hannan, Palm Beach Gardens, Fla; Joseph Ralph Frattarola,l-lightstown, NJ.

[73] Assignee: RCA Corporation, New York, N.Y. [22] Filed: Oct. 12, 1972[21] Appl. No.: 296,861

[52] US. Cl. 350/35, 352/233 [51] Int. Cl. G02b 27/00 [58] Field ofSearch. 350/35, 162 R, 162 S, 162 F, 350/162 2?; 178/63; 179/100.3 V;250/219 DD', 352/85 H, 232, 233

[56] References Cited UNITED STATES PATENTS 3,565,978 2/1971 Folger etal 350/3.5 3,619,025 11/1971 Hannan 350/35 3,430,966 4/1969 Gregg179/1003 V Primary Examiner--Ronald J. Stern Attorney, Agent, orFinn-Glenn H. Bruestle; Birgit E. Morris; Arthur 1. Spechler ABSTRACT Adouble-sided, holographic replica comprises a web of transparent,therrno lastic, sheet material having a separate series ofoff-axis-recorded, phase-modulated holograms pressed into each of theopposite surfaces, respectively, of the web. Oppositely disposed (frontand rear) holograms are such as to provide angularly displacedreconstructed images, respectively, when a monochromatic read-out beamis transmitted through the web. The double-sided holographic replica ismade by first pressing the web and a first holographic master between afirst pair of heated calender rollers to form one replica on one surfaceof the web, and then pressing the web, while thermoplastically adheredto the first master, and a second holographic master between a secondpair of heated calender rollers to form another replica on the othersurface of the web. The first and second masters are subsequently cooledand separated from the web.

6 Claims, 7 Drawing Figures PATENTEU 51974 3. 790.245

.SfltEIlUfZ TRANSPARENCY REDUNDANCY 24 MEANS 26 28 LASER L J M 34 g-READ-OUT BEAM AXIS 52 R L 54 BEAM AXIS y 'Tv Z 44 CAMERA PAIENTEU FEB 5I974 SHEEI 2 0F 2 1 DOUBLE-SIDED IIOLOGRAPIIIC REPLICAS BACKGROUND OFTHE INVENTION This invention relates generally to holograms, and, moreparticularly, to holographic replicas formed in the opposite surfaces oftransparent sheet material, and to a method of making the same.

A hologram is a recording in the form of an interference fringe patternof all the information in a wave front of light obtained from an objectthat is illuminated with spacially monochromatic light.

By the term phase-modulated holograms, as used herein and in theappended claims, is meant holograms made by the technique of phaseholography, which comprises, for example, projecting interferencepatterns, produced by the phase delay of two light beams, onto arecording medium that is developed to provide relief patterns, in amanner well known in the art.

It has been proposed to prepare phase-modulated holograms on aphotoresist coated on a web substrate. When the photoresist is exposeddirectly with a holographic image of an object, and the photoresist isdeveloped, the resulting phase-modulated hologram is termed an originalhologram. A metal holographic master can be prepared from the originalphase-modulated hologram by electroless plating and/r electroplatingtechniques to reproduce the holographic information on the metal master,in a manner known in the art, The advantage of preparing a metal masteris that many holographic replicas canbe pressed from a single metalmaster because of its dimensional stability.

It has also been proposed to make holographic replicas by pressingholographic relief patterns from a holographic master onto one surfaceof thermoplastic sheet material. Because of the prior-art method ofrecording and playing back phase-modulated holograms, however, it hasnot been possible previously to make a practical holographic replica bypressing holographic information onto the opposite surfaces of the samethermoplastic sheet material. The reason for this is that prior-artmethods of making phase-modulated holograms comprised exposing therecording medium with a light interference pattern wherein the planedefined by the object beam and the reference beams was disposedperpendicularly to the longitudinal axis of the recording medium. By theterm longitudinal axis," as used herein is meant the imaginary axis ofthe hologram that is perpendicular to the plane defined by the objectand reference beams in priorart holograms, regardless of the actualposition of this axis to the horizontal. If these prior-art reliefpatterns (replicas) of holograms were pressed onto the opposite surfacesof a transparent tape, for example, the reconstructed images of both thefront and rear holograms would be superimposed on each other when theholographic replicas were played back, producing an undesirable result.

We have found that when the longitudinal axis of a holographic recordingmedium is disposed at an ob lique angle (other than 90 or 0) to theplane formed by the object and reference beams, an off-axis,phasemodulated hologram can be made by a method of holographicrecording, herein referred to as a tilted off-axis recording, describedin U. S. Pat. No. 3,6l9,052, issued on Nov. 9, 1971, to W. .I. Hannan,the coinventor herein. If holographic masters are made from tiltedoffaxis, phase-modulated hologram recording media, then a transparentsheet material can be pressed on opposite surfaces with such masters,and the resulting doublesided holographic replica can be played backwith a transmitted, monochromatic, read-out beam so that the projected,reconstructed image from the holographic replica on one surface of thesheet material is angularly displaced from, and does not interfere with,the projected reconstructed image from the holographic replica on theopposite surface of the sheet material.

SUMMARY OF THE INVENTION A novel information recording medium comprisesa transparent sheet material with a separate relief pattern on each ofthe opposite surfaces of the sheet material.

In a preferred embodiment of the novel recording medium, a differenttilted off-axis holographic relief pattern is formed on each of theopposite surfaces of a web of transparent sheet material. Oppositelydisposed (front and back) relief patterns are such as to provideangularly displaced reconstructed images, respectively, when amonochromatic read-out beam is transmitted through the sheet material.

In another embodiment of the novel recording medium, the relief patternon at least one of the opposite surfaces of the sheet material is atilted off-axis hologram.

The novel method of making double-sided-reliefpattern replicas comprisespressing a relief pattern, as from a first master of a tilted off-axisrecording hologram, onto one surface of a thermoplastic sheet materialat a temperature below the flow temperature of the thermoplasticmaterial but sufficient to cause the sheet material to adheretemporarily to the first master, and then pressing a relief pattern, asfrom another master, onto the other side of the thermoplastic sheetmaterial while the sheet material is still adhered to the first master.The thermoplastic sheet material is next cooled and the masters areseparated therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a fragmentary, perspectiveview of a novel, double-sided, holographic replica, showing a separateseries of replicated holograms on opposite surfaces, respectively, of aflexible, transparent web of thermoplastic sheet material;

FIG. 2 is a schematic drawing of an arrangement of apparatus forrecording a tilted off-axis hologram, corresponding to spacialinformation on a transparency, onto a hologram recording medium;

FIG. 2a is a cross section of the diagram shown in FIG. 2, taken alongthe line 2a2a and viewed in the direction of the arrows;

FIG. 3 is a schematic diagram of an arrangement of apparatus for playingback a tilted off-axis recorded hologram to reconstruct an image ofspacial information on a photosensitive imaging surface of a televisioncamera;

FIG. 4 is a schematic diagram of apparatus used in the method of makingdouble-sided holographic replicas on a web of transparent, thermoplasticsheet material;

FIG. 5 is a cross section of a portion of the apparatus shown in FIG. 4,taken along the line 5--5 and viewed in the direction of the arrows; and

FIG. 6 is a cross section of a portion of the apparatus shown in FIG. 4,taken along the line 66 and viewed in the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of thedrawing, there is shown a fragmentary perspective view of one embodimentof a novel information recording medium in the form of a double-sidedholographic replica 10. The holographic replica 10 comprises a web 12 oftransparent sheet material with one series of phase-modulated holograms14 pressed into one surface I6 of the web I2 and another series ofphase-modulated holograms 18 pressed into an opposite surface 20 of theweb 12.

The holograms I4 and 18 are relief patterns that are impressed in thesurfaces 12 and 20 of the web 12, respectively, by metal masters in anovel method hereinafter to be explained. Each of the holograms l4 and18 may be considered to have a longitudinal axis 21 extending in theintended direction of travel of the web 12. The web 12 is preferably ofthermoplastic polymeric sheet material to provide a stable, flexible,transparent, and isotropic medium for the holograms l4 and 18 pressedtherein. The web 12 is also of an optical quality and transparency atleast equal to that of window glass.

A web of flexible, transparent, cast-vinyl, thermoplastic material,having a thickness of between 1 and 6 mils, has been found satisfactoryfor the web 12. Cast vinyl has been found superior to calendered vinylin that the former provided better optical clarity and less noise andstress patterns under polarized light than the latter. A typical web 12for storing information to be played back through a television receiveris about onehalf inch wide with holographic information thereon in aseries of relief patterns of about X mm. However, a web 12 of almost anywidth and length can be used provided that the holo-graphic informationcan be pressed therein. A typical web I2 is about 2 mils thick, and thepeak to valley depth of the surface corrugations that form thephase-modulated holograms l4 and I8 (holographic contours) is on theorder of between 5 and 2,000A.

Inasmuch as the holographic replica 10 will generally be used inconjunction with a web transport system, the web 12 should have physicalcharacteristics such that it is not deformed under the tension of thetransport system. For example, the web 12 should have a tensile strengthof about 5,000 psi and a maximum elongation before rupture (at 25C) of 5percent. The flow temperature of the thermoplastic web 12, of the typedescribed, is 155C i 1C. A transparent, thermoplastic, vinyl materialthat has been found satisfactory for the holographic replica 10 isplasticized, polyvinyl chloride-vinyl acetate copolymer made by theCadco Film Division of Cadillac Plastic and Chemical Co., Inc., Detroit,Mich., formulation No. C-l02.

The holograms l4 and 18 are relief patterns impressed upon the web 12 bymetal masters of tilted offaxis recorded holograms. Referring now toFIG. 2 of the drawing, there is shown an arrangement of apparatus formaking a tilted off-axis recorded hologram. A holographic recordingmedium 12a, which can be in the form of a length of substrate tape orweb coated with a suitable photoresist. The length of the web (recordingmedium 12a) lies in a plane perpendicular to the paper but extends in adirection that is oblique to the plane defined by the object andreference beam axes 22 and 32; that is, the longitudinal axis 12a of therecording medium 120 forms an angle other than or zero degrees with theplane 23 that contains the object and reference beams, as shown in FIG.2a. A laser 24 emits a beam 25 of optical energy of a first givenwavelength M. A partially reflecting mirror 26, called a beam splitter,divides the laser output beam 25 into two component beams 28 and 30.Beam 30, after reflection from a mirror 31, forms a reference beam,directed along the reference beam axis 32, that impinges upon theholographic recording medium 12a.

The light beam 28 is passed through redundancy means 34, which may be adiffuser, or preferably a phase grating of the type disclosed in U. S.Pat. No. 3,650,595. The optical energy emerging from the redundancymeans 34 illuminates a transparency 36 having a spacial pattern ofinformation thereon to be reproduced. If a Fresnel hologram is to bemade, a convex lens 38 may be omitted and the optical energy emergingfrom the transparency 36 may be applied directly to the holographicrecording medium 120. If, however, a F raunhofer hologram is to be made,the lens 38 is disposed in front of the transparency 36 at a distanceequal to the focal length of the lens 38 so that the transparency 36lies in the focal plane of the lens 38. In the latter case, the outputof optical energy from the lens 38 is applied as the information beamabout the information beam axis 22 to the holographic recording medium120. The developed holographic recording medium provides a tiltedoff-axis hologram record 12b (FIG. 3) which can have a resolution ofabout 1 micron.

Referring now to FIG. 3, there is shown an arrangement of apparatus toplay back the developed tilted offaxis hologram record 12b. A laser 42produces a readout beam of optical energy disposed about a read-out axis44 at a second given wavelength A, which may be the same as, ordifferent from, the recording wavelength M. The read-out beam impingesupon the hologram record 12b at an angle of incidence x, with respect tothe normal 46 to the surface of the hologram record 12b.

The essential difference between the play-back technique of the tiltedoff-axis recorded hologram 12b and the prior-art play-back technique,other than the fact that the tilted off-axis hologram record 12b wasrecorded by a different technique, is in the orientation of the hologramrecord 12b with respect to the read-out beam axis 44 and a real imagebeam axis 48. The image represented by the hologram 12b is projectedalong the real image beam axis 48 and focused onto, for example, asensitized surface 50 in a TV camera 52, by means of a lens 54. Thesurface 50 is at the focal distance of the lens 54; and the real imagebeam axis 48 forms an oblique angle x with the normal 46 to the surfaceof the hologram record 12b. The plane defined by the readout beam andreal image beam axes 44 and 48, respectively, is oblique to thelongitudinal axis of the hologram 12b. With the arrangement of apparatusshown in FIG. 3, scratches on the surface of the hologram record 12bwill not be superimposed upon the reconstructed real image projectedonto the surface 50 of the camera 52, as discussed in greater detail inthe aforementioned U. S. Pat. No. 3,619,052.

If the hologram record 12b in FIG. 3 were a doublesided tilted off-axisrecorded holographic replica, such as the replica 10 described inconjunction with FIG. 1, the holographic replica 10 could be played backby the play-back arrangement shown in FIG. 3 so that only one series ofholograms, such as the holograms 14 on one surface, is imaged on thesurface 50 of the camera 52. To image the other series of holograms,such as the holograms 18 on the other surface 20 of the holographicreplica 10, onto the surface 50 of the camera 52 without interferencefrom the holograms 18, the holographic replicas would merely have to bereversed.

Referring now to FIG. 4 of the drawing, there is shown apparatus for themethod of making the doublesided holographic replica 10 on the web 12 oftransparent thermoplastic sheet material. The web 12 is unrolled from asupply roll 56 and directed: over a flanged guide roller 58, between afirst pair of calender rollers 60 and 62, between a second pair ofcalender rollers 64 and 66, under a flanged guide roller 68, and rolledup on a take-up roll 70. The rollers 62 and 64 are heated, by anysuitable means (not shown) and coated with layers 72 and 74,respectively, of silicone rubber. The rollers 60 and 66 are also coveredwith layers 76 and 78, respectively, of silicone rubber.

A first metal master 80 ofa series of tilted off-axis recordedholograms, such as of the holograms 14 on the surface 16 of the web 12illustrated in FIG. I, is formed in an endless loop (as by welding, forexample,) and disposed around rollers 82, 83, 84, 85, and 86 forrotation thereabout in the direction of the arrow 88 by any suitablemotive means. A portion of the master 80 is also disposed between thefirst pair of calender rollers 60 and 62 and the second pair of calenderrollers 64 and 66.

A second metal master 90 of a second series of tilted off-axis recordedholograms, such as of the holograms 18 on the surface 20 of the web 12,shown in FIG. I, is also formed in an endless loop (as by welding, forexample.) and disposed around rollers 91, 92, 93, 94, 95, and 96, forrotation thereabout in the direction of the arrow 98 by any suitablemotive means. The master 90 passes between only the second pair ofcalender rollers 64 and 66. A blower 100 for blowing cool air onto themetal master 90 is disposed between the calender roller 64 and theroller 95. Another blower 102 is disposed between the calender roller 66and the roller 85 for blowing cool air onto the metal master 80.

The method of forming a separate series of replicas of different reliefpatterns in each of the opposite surfaces of the web 12 will now bedescribed with the aid of the apparatus shown in FIG. 4. The web 12 isunrolled from the roll 56, guided over the flanged guide roller 58 andonto the silicone rubber surface of the calender roller 60. The lowersurface 16 (FIG. 5) of the web 12 is brought into contact with the metalmaster 80 between the first pair of calender rollers 60 and 62. Thetemperature of the heated roller 62 is preferably about 130C, that is,below the flow temperature of the thermoplastic web 12. The pressurebetween the first pair of calender rollers 60 and 62 can range between10 and 200 psi. The web 12 is fed through the first pair of calenderrollers 60 and 62 at a speed of about 4 inches per second. Under theaforementioned conditions, the metal master 80 presses impressions ofthe series of holograms 14 onto the surface 16 of the web 12, and theweb 12 is temporarily thermoplastically adhered to the metal master 80so that it cannot move with respect to it.

Next, the metal master 80, with the web 12 adhered to it, is moved overthe silicone rubber surface of the roller 66 and the second metal master90 is brought into contact with the surface 20 of the web 12 between thesecond pair of pressurized calender rollers 64 and 66. The calenderroller 64 is heated to a temperature of preferably 100C, and thepressure between the rollers 64 and 66 is in the range between 10 and200 psi. Under these conditions, a second series of holograms 18 isimpressed onto the surface 20 of the web 12. The metal masters and arenow cooled by cool air from the blowers 102 and 100, respectively, to atemperature below 30C. At this temperature the web 12 no longer adheresto the metal masters 80 and 90 and is moved under the flanged guideroller 68 and wound up on the take-up roll 70.

The thermoplastic web 12 should never be heated to the flow temperaturethereof. The heat applied by the calender rollers, however, may be inthe range from about 0.3 to 0.85 times the flow temperature of the web12.

We have found that an improved double-sided holographic replica 10 isformed when the series of holograms for the opposite surfaces of the webI2 are applied in a sequential manner, that is, with two pairs ofcalender rollers, as shown in FIG. 4, rather than with only one pair,because it is relatively difficult to guide the web 12 simultaneouslybetween two metal masters without unwanted slippage. Thus, by firstguiding the web 12 onto the silicone rubber surface of the calenderroller 60 and impressing the holograms 14 onto the surface 16 of the web12 by the master 80, the web 12 adheres to the master 80 so that boththe web 12 and the master 80 can be guided between the second pair ofcalender rollers 64 and 66 without any possible movement between the web12 and the master 80. Under these conditions, the holograms 18 can beimpressed on the surface 20 of the web 12 with accuracy and in the exactpositions desired. The depth of the holographic impressions (reliefpatterns) may vary between 5 and 2,000A.

None of the aforementioned parameters is critical, and it is possible tovary them somewhat and still provide high-quality replicas. Thetemperature and pressure parameters will vary with the thickness of thethermoplastic web used, a somewhat higher temperature being necessarywhen a thicker web is used. We have also found that it is preferable toheat the calender roller in contact with the master rather than to heatthe roller in contact with the replica (substrate) web. This latterprocedure results in better reproduced holographic replicas. Also, theuse of yieldable silicon rubber on the calender rollers tends to reduceunwanted slippage and provides improved holographic replicas.

The use of cast vinyls for the web 12 proved superior to calendered orextruded materials.

The blowers and 102, which may be conduits having a plurality ofopenings through which compressed air can pass, are disposed parallel tothe masters 80 and 90, respectively, along a substantial length thereofso as to cool the masters sufficiently and thereby minimize distortionor elongation of the web when separating the web from the masters.Although it is preferable to cool the masters 80 and 90 to at least 30Cbefore separating them from the web 12, the temperature of the masters80 and 90 should be below 50C before separation is attempted.

The web 12 should preferably have a maximum elongation prior to ruptureof percent at room temperature. Webs having greater elongation prior torupture tend to have the replicated relief patterns distorted to such adegree as to degrade the image during play back. Also, the web should beflexible and capable of being wound on reels. While the examplesdescribed herein relate to pure vinyl material, the thermoplasticmaterial may be in the form of a laminated tape, such as vinyl materialcoated on a Mylar base, if extra strength is desired.

Although the masters 80 and 90 are preferably made of nickel, othersuitable metals or plastics may be employed.

Alternatively, instead of replicating a separate tilted off-axisrecorded holographic series of holograms on each of the oppositesurfaces of the thermoplastic web 12, one of the surfaces can have arelief pattern of audio information pressed therein rather than thesecond series of holograms.

I claim:

I. An information recording medium consisting of substantiallyhomogeneous transparent sheet material having formed in each of theopposite surfaces thereof a separate relief pattern, each of saidseparate relief patterns comprising a tilted off-axis recordedtransmission hologram formed by object and reference beams which definea plane oblique to the longitudinal axis of the hologram, saidoppositely disposed relief patterns providing angularly displacedreconstructed images when a monochromatic read-out beam is transmittedthrough the sheet material.

2. An information recording medium as-dcscribed in claim 1, wherein saidsheet material is a web of thermoplastic material. 3. An informationrecording medium as described in claim 1, wherein said relief patternshaving a depth in the range between 5 and 2,000A. 4. An informationrecording medium as described in claim 1, wherein said sheet materialhas a thickness of between about 1 and 6 mils, and said holograms have aresolution of about one micron. 5. An information recording medium asdescribed in claim 1, wherein said sheet material is cast vinyl, andsaid holograms formed in said opposite surfaces of said sheet materialare opposite each other. 6. An information recording medium as describedin claim 5, wherein said cast vinyl sheet material has an elongationbefure rupture of less than, or equal to, 5 percent at 25C.

i I i t

1. An information recording medium consisting of substantiallyhomogeneous transparent sheet material having formed in each of theopposite surfaces thereof a separate relief pattern, each of saidseparate relief patterns comprising a tilted off-axis recordedtransmission hologram formed by object and reference beams which definea plane oblique to the longitudinal axis of the hologram, saidoppositely disposed relief patterns providing angularly displacedreconstructed images when a monochromatic read-out beam is transmittedthrough the sheet material.
 2. An information recording medium asdescribed in claim 1, wherein said sheet material is a web ofthermoplastic material.
 3. An infoRmation recording medium as describedin claim 1, wherein said relief patterns having a depth in the rangebetween 5 and 2,000A.
 4. An information recording medium as described inclaim 1, wherein said sheet material has a thickness of between about 1and 6 mils, and said holograms have a resolution of about one micron. 5.An information recording medium as described in claim 1, wherein saidsheet material is cast vinyl, and said holograms formed in said oppositesurfaces of said sheet material are opposite each other.
 6. Aninformation recording medium as described in claim 5, wherein said castvinyl sheet material has an elongation befure rupture of less than, orequal to, 5 percent at 25*C.